1. Field of the Invention
The present invention relates to an optical modulator used for optical communications and optical data processing, and more specifically to the structure of an optical modulator which exhibits excellent modulation velocity and modulation efficiency.
2. Prior Art
In a semiconductor superlattice using such semiconductors as GaAs and GaAlAs, the excitons behave like two-dimensional hydrogen atoms due to the quantum size effect. In this case, the Bohr radius is about one-fourth and the binding energy is about four times as great compared with the case when the excitons behave like three-dimensional hydrogen atoms in the semiconductor bulk crystal. Therefore, the excitons that are observed only at low temperatures in the bulk crystal, can be observed in the superlattice structure even at room temperature (see D. A. B. Miller et al "Large Room-Temperature Optical Nonlinearity in GaAs/Ga.sub.1-x Al.sub.x As Multiple Quantum Well Structures", Appl. Phys. Lett. Vol. 41, pp. 679-681, 1982).
If an electric field is applied to the excitons, the absorption spectra change due to the Stark effect. There has been proposed an optical modulator which utilizes this principle (see T. H. Wood et al., "High-Speed Optical Modulation with GaAs/GaAlAs Quantum Wells in a p-i-n Diode Structure", Appl. Phys. Lett. Vol. 44, pp. 16-18, 1984).
FIG. 1 illustrates the structure of a conventional optical modulator, wherein reference numeral 1 denotes an n-type GaAs substrate, 2 denotes an etching stopping layer composed of n-type GaAlAs, 3 denotes an n-type superlattice contact layer, 4 and 6 denote undoped superlattice buffer layers, 5 denotes an undoped superlattice active layer, 7 denotes a p-type superlattice contact layer, 8 denotes a p-type GaAlAs contact layer, 9 denotes an electrode of the n side, and 10 denotes an electrode of the p side.
The incident light (signal light) 11 is modulated as it passes through the undoped superlattice active layer to which a bias voltage is applied, and goes out therefrom as denoted by 12. In this case, the exciton consists of an electron and a positive hole that exist in a quantum well, and is very likely to produce recombination emission and decay. The recombination emission which is overlapped on the signal light deteriorates the signal-to-noise ratio of the optical demodulator. Further, a photocurrent produced by the signal light causes the applied voltage to drop substantially, so that the modulation efficiency decreases.
Methods have also been announced to separate the electrons and the positive holes in real space (see Staggered-Lineup Heterojunctions as Sources of Tunable Below-Gap Radiation: Operating Principle and Semiconductor Selection, IEEE Trans. Vol. Ed-4, pp. 20-22, 1983, and E. J. Caine et al., "Staggered-Lineup Heterojunctions as Sources of Tunable Below-Gap Radiation: Experimental Verification", Appl. Phys. Lett. Vol. 45, pp. 1123-1125, 1984). Even in these cases, however, the electrons and the positive holes are not sufficiently separated, and the above-mentioned defects are not overcome.
FIG. 3 is a diagram of energy bands of these cases. The electron 17 and the positive hole 18 are confined in a triangular potential well formed by the application of a bias voltage 24. However, the electron and the positive hole are likely to recombine as denoted by 19 and 20 due to the tunnel effect, since the barrier is not so high.